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    Chapter 8 - Alkene Reactivity

    Uploaded by

    Simran Dhunna
    1) The document summarizes reactions and synthesis of alkenes, including elimination reactions to form alkenes, halogenation, halohydrin formation, hydration, oxymercuration, hydroboration, hydrogenation, dihydroxylation, diol cleavage, and ozonolysis. 2) Key reactions covered include acid-catalyzed and base-catalyzed elimination, halogenation by HX and X2 addition, regioselectivity of HX addition, anti-addition of X2, Markovnikov selectivity of hydration and oxymercuration, syn-addition of hydroboration, and stereospecificity of hydrogenation and dihydroxylation.

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    Chapter 8 - Alkene Reactivity

    Uploaded by

    Simran Dhunna
    59 views23 pages
    1) The document summarizes reactions and synthesis of alkenes, including elimination reactions to form alkenes, halogenation, halohydrin formation, hydration, oxymercuration, hydroboration, hydrogenation, dihydroxylation, diol cleavage, and ozonolysis. 2) Key reactions covered include acid-catalyzed and base-catalyzed elimination, halogenation by HX and X2 addition, regioselectivity of HX addition, anti-addition of X2, Markovnikov selectivity of hydration and oxymercuration, syn-addition of hydroboration, and stereospecificity of hydrogenation and dihydroxylation.

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    1) The document summarizes reactions and synthesis of alkenes, including elimination reactions to form alkenes, halogenation, halohydrin formation, hydration, oxymercuration, hydroboration, hydrogenation, dihydroxylation, diol cleavage, and ozonolysis. 2) Key reactions covered include acid-catalyzed and base-catalyzed elimination, halogenation by HX and X2 addition, regioselectivity of HX addition, anti-addition of X2, Markovnikov selectivity of hydration and oxymercuration, syn-addition of hydroboration, and stereospecificity of hydrogenation and dihydroxylation.

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    59 views23 pages

    Chapter 8 - Alkene Reactivity

    Uploaded by

    Simran Dhunna
    1) The document summarizes reactions and synthesis of alkenes, including elimination reactions to form alkenes, halogenation, halohydrin formation, hydration, oxymercuration, hydroboration, hydrogenation, dihydroxylation, diol cleavage, and ozonolysis. 2) Key reactions covered include acid-catalyzed and base-catalyzed elimination, halogenation by HX and X2 addition, regioselectivity of HX addition, anti-addition of X2, Markovnikov selectivity of hydration and oxymercuration, syn-addition of hydroboration, and stereospecificity of hydrogenation and dihydroxylation.

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    of 23

    Chapter 8

    Topics

    Alkenes: Reactions and Synthesis

    (not Simmons-Smith reactions or epoxidation, not 8.10 and not 8.11)

    Synthesis via elimination reactions Halogenation and halohydrin formation Addition of water to alkenes(oxymercuration, hydroboration-oxidation) Carbene reactions (:CCl2) Hydrogenation (H2/cat.) Dihydroxylation (OsO4), diol cleavage with IO4Ozonolysis Stereochemistry of alkene addition

    Synthesis of alkenes via elimination reactions: Base elimination


    Br
    K OH , ethanol

    + Br- + H20

    OH

    Acid-catalyzed elimination
    CH 3 OH H 2 SO 4 , H 2 O THF

    + H20

    THF =
    O

    a solvent that is miscible with water


    2

    Quick review: Addition of HX (X=Cl, Br, I) to alkenes

    CH3

    HBr ether

    Br CH3

    This reaction is regiospecific

    M arkovnikov

    achiral

    CH3
    Non-Markovnikov

    CH3 Br

    CH3 Br

    CH3 Br

    Br

    The reaction goes through the more stable carbocation.


    3

    When both carbocations have similar stability:

    CH3 CH2CH3 HBr Br CH3 H CH2CH3 H Br CH3 CH2CH3 Br H CH3 H CH2CH3 Br CH3 CH2CH3 Br H CH3 CH2CH3 Br H CH3 CH2CH3 H CH3 CH2CH3

    Br CH3 H CH2CH3

    Br

    All of the possible stereoisomers are formed


    4

    Addition from the top or bottom


    H

    H
    Br

    CH3

    cis

    CH3 CH3

    CH3

    Br
    carbocation intermediate
    Br

    H CH3 CH3

    trans

    Halogenation: Addition of X2

    What is a possible mechanism?

    H C H C

    Cl2 ClCH2CH2Cl

    Br Br2

    Br

    From H-X addition:

    By this mechanism, should see 50:50 mixture of cis and trans. In reality, only the trans isomer is observed.

    Reaction occurs with anti stereochemistry (atoms add from opposite faces of 7 the double bond.

    Halogenation: Addition of X2

    Bromonium ion shields one face of ring. Only the opposite face is available. ANTI addition

    Halohydrin formation

    Br2 / H2O

    Br OH

    Br--Br

    Br
    Who wins? [H2O] >> [Br-] + Br+ OH2
    9

    Halohydrin formation

    Br2 / H2O

    Br OH
    + H 3 O+

    Br--Br

    Br

    Br OH2

    + Br-

    + OH2

    [H2O] >> [Br-]


    10

    Halohydrin formation

    Cl2/H2O

    Br2/H2O

    I2/H2O

    Bond from bromonium to the more substituted carbon is weaker. More + charge on more substituted carbon
    H3C

    Br +

    H3C H HO

    Br

    Br

    Br

    OH 2 OH 2

    with deprotonation step (next slide)


    HO H

    H3C H3C

    H Br H3C
    +
    11

    Br

    Paying attention to details when you write out the mechanism


    Br + H3C HO Br

    H3C OH2

    H3C H O
    +

    Br

    H H OH2

    12

    Halohydrin formation (2)

    OH CH3 CH3

    OH CH3

    +
    H Br Br H

    anti addition Regiospecific: the OH (from H2O as the nucleophile) becomes attached to the site of the more stable carbocation. None of the other isomers are formed.
    OH

    Br CH3

    13

    Bromohydrin formation with NBS


    Br2 source

    nucleophile solvent

    NBS reacts slowly with water to produce a low constant level of Br2. This is a safer way of running this reaction. Note ring doesnt react.
    O O

    O CH3 S CH3

    Br H2O/DMSO

    + Br2

    DMSO helps to dissolve the reactants


    14

    Which product will result from the following reaction?


    Cl2 CH2Cl2

    1.

    2.

    3. 4. 5.

    Which of the compounds will result from the reaction of NBS/H2O/DMSO with 1-methylcyclopentene?
    1. 2.

    3.

    4.

    5.

    Hydration of alkenes

    In industry
    H 3PO4, pH 0
    + H 2O

    OH Ethanol

    250 C

    goes through HX addition mechanism

    not useful on small scale in the laboratory

    In living cells
    COOH
    + H 2O

    Fumarase, pH 7 37 C HOOC
    *

    OH COOH

    HOOC

    17

    Electrophilic Addition Reactions Review


    Addition of H-X
    H-Br C C C H C Br

    C H

    + Br -

    18

    Electrophilic Addition Reactions


    Addition of X-X

    Br Br-Br C C C C Br

    Br C C

    + Br -

    19

    Electrophilic Addition Reactions


    Halohydrin Formation

    Br Br-Br C C H2O OH C C

    + H 3 O+

    Br

    Br C C

    + H 2O

    C O H

    O H H

    20

    10

    Electrophilic Addition Reactions


    Acid-Catalyzed Hydration

    O H H H

    H H2SO4 C C H2O OH C C

    + H 3 O+

    C H

    + H 2O

    C O H H H O H

    21

    Hydration reactions in the Lab


    _______________
    CH3 1. Hg(OAc)2, H2O 2. NaBH4 OH CH3

    Markovnikov

    _______________
    CH3 1. BH3 2. H2O2 /OHH CH3 H OH OH H CH3 H

    Two-step processes

    "Non-Markovnikov" and Syn addition


    22

    11

    Oxymercuration
    O
    O

    O
    O

    O +

    H
    H

    Hg
    O

    HO
    O

    +
    O

    Hg
    O

    Hg

    O
    H

    + O

    Hg O

    Attack on more substituted carbon (it has more positive charge character)

    H O

    O
    O

    NaBH4
    Hg O

    + Hg + 2 HOAc

    Markovnikov, but not stereospecific

    The reaction stops here but this product is not isolated

    23

    Hydroboration

    Borane is a good electrophile with an empty p-orbital

    24

    12

    Hydroboration: syn addition

    B becomes attached to the less substituted carbon

    25

    NaOH, H2O2
    H H H BH2 H H H OH

    The C-B bond is replaced with a C-O bond.


    26

    13

    What is the product of reaction of 1-methylcyclohexene with Hg(OAc)2/H2O/THF followed by NaBD4? (D = 2H)
    1. 2.

    3.

    4.

    5.

    Hydrogenation:
    Addition of H2 gas to alkenes gives alkanes (or cycloalkanes).
    H H2 catalyst H

    Both H atoms add to the same face of alkene, SYN addition. Catalyst: often Pd/C or PtO2 (Adams catalyst)

    28

    14

    Hydrogenation:
    The double bond is hydrogenated or reduced by the addition of H2. Other multiple bonds are usually unaffected by these conditions.

    29

    Which compound is the best starting material for hydrogenation to decalin shown below?
    ? H2 (< 1atm) H Pd/C H

    1.

    2. 3.

    4.

    5.

    15

    ______________
    1) O sO 4 2) N aH SO 3 /H 2 O

    OH

    OH
    Anti addition products are not formed

    CH 3 H

    1)

    OsO 4

    OH CH3 H OH OH CH3 H OH Syn

    OH CH3 H OH OH CH3 H OH
    31

    2) NaH SO 3/H 2O

    Markovnikovs rule not needed

    Mechanism: Why Syn?

    O Os O

    O O

    O O NaHSO3/H2O HO

    Os O HO

    Os OH OH

    32

    16

    Diol cleavage with IO4-

    O HO HO H OH CH 3
    +

    O I O CH 3

    O HIO 4 H

    O O H CH 3

    33

    Alkene Cleavage
    C=C bonds can be completely broken. Two methods: Diol formation, followed by cleavage with HIO4 Ozonolysis (using ozone, O3) Permanganate Oxidation (using KMnO4, H+) Products are C=O containing compounds.

    34

    17

    ___________

    Recall:

    1) OsO4 2) NaHSO3/H2O
    35

    Typical questions

    What are A. B, D?
    36

    18

    Alkene cleavage with _______________

    If there is an H on the double bond, the C gets oxidized to -COOH


    37

    Substitution of Ozonolysis Alkene Products Tetra 2 ketones

    Permanganate Products 2 ketones

    Tri Di (same carbon) Di (different carbon) Mono

    1 ketone, 1 aldehyde 1 ketone, HCHO 2 aldehydes 1 aldehyde, HCHO

    1 ketone, 1 carboxylic acid 1 ketone, CO2 and H2O 2 carboxylic acids 1 carboxylic acid, CO2 and H2O

    38

    19

    Which of the following will react with periodic acid (HIO4) to give an aldehyde?
    1. 2.

    3.

    4.

    5.

    Which alkene would produce two identical ketones when reacted with KMnO4/H+?
    1. 2.

    3.

    4.

    5.

    20

    Addition of Carbenes to Alkenes:


    Carbene = R2C:, neutral, 6 electrons in valence shell, VERY reactive

    41

    Dichlorocarbene
    Acidic proton

    - ClHCCl3 + HO- HOH + -CCl3 :CCl2 Cl Cl Cl Cl

    Strong base

    chloroform

    42 42

    21

    Stereochemistry of Reactions (8.12 and 8.13)


    A racemic mixture is produced

    Carbocation is planar (with plane of symmetry) Br- can attack equally well from top and bottom.

    43 43

    Addition of Br2

    44

    22

    Addition of Br2

    45

    Addition of HBr to Chiral Alkene

    46

    23

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